Days after Lockheed Martin revealed its network had been breached by hackers, two more defense contractors were hit by cyber-attacks.

Another defense contractor appears to have been hit by a cyber-attack, and a leaked memo indicates the executives believe attackers used information stolen from RSA Security earlier this year. If true, RSA's SecurID technology may be irrevocably compromised.

Attackers hit major defense contractor L-3 Communications Holdings by spoofing passcodes from a cloned RSA SecurID token, Reuters reported May 27. The attackers may have used a similar method to target another defense contractor, Lockheed Martin, on May 21. The second largest U.S. defense contractor Northrop Grumman may also have been hacked, as the company shut down remote access to its network without warning on May 26, according to Fox News.

L-3 Communications was formed out of ten business units that had been spun off by Lockheed prior to its merger with Martin Marietta back in 1995. L-3 is a major supplier of communication, intelligence, surveillance and reconnaissance technology to the Department of Defense.

"L-3 Communications has been actively targeted with penetration attacks leveraging the compromised information," an L-3 executive wrote in an internal April 6 memo obtained by Wired Threat Level.

It's not clear from the internal e-mail whether attackers managed to actually break into L-3 networks, or if they were detected in the midst of the attack. The memo also did not specify exactly why or how L-3 came to the conclusion that the SecurID two-factor authentication system was at fault. A L-3 spokesperson just said the company takes security seriously and that the incident has been resolved.

RSA Security admitted March 17 that cyber-attackers had breached its network and obtained "information relating to the SecurID technology." The company has steadfastly refused to publicly discuss exactly what was stolen or when the breach actually occurred. RSA later disclosed that it had been hit by a phishing e-mail exploiting a zero-day vulnerability in Adobe Reader.

At the time, RSA executive chairman Art Coviello said the stolen information "could potentially be used to reduce the effectiveness of a current two-factor authentication implementation as part of a broader attack."

For someone to break into a SecurID-protect network, the attacker would need at least one employee's username and pass code as well as have some idea of which services that employee had access to.